The method disclosed herein relates to improvements in the well treating method disclosed in U.S. Pat. No. 4,250,965 which issued Feb. 17, 1981 to Ben W. Wiseman, Jr., the disclosure of which is incorporated herein by reference in its entirety. The Wiseman patent discloses a method of stimulating primary production of a well wherein liquid carbon dioxide evaporated to form anhydrous gaseous carbon dioxide which was delivered to permeate through the strata to react with connate water to make carbonic acid. The well was shut in for a period of time enabling the carbonic acid to react with the rocks in the strata to increase the permeability of the strata and thereby stimulate production of the well.
Corrosive acidizing agents such as hydrofluoric, hydrochloric, acetic, and carbonic acids are often used for stimulating the production from an oil or gas well. Conventionally, these acidizing agents are liquids, which are pumped at high pressure into the well to react with rocks in the oil or gas bearing strata in an attempt to increase the permeability of the strata. When the corrosive acids are being pumped down the production tubing or casing, they react to some degree with the materials used to make the tubing or casing even when an inhibitor is mixed with the acid.
In some cases the formation may be damaged when the liquid is pumped into the formation at high pressure. Clay particles tend to swell and become disassociated from rock in the formation which may reduce permeability. The creation of an oil/water emulsion and the entrapment of water by surface tension also reduce permeability.
Conventional acidizing processes generally require the use of substantial quantities of water and equipment to flush and swab the well to minimize damage to the tubulars in the well. Recovery and disposal of the water to prevent ecological damage is expensive and time consuming. Further, water introduced and left in the formation may damage the reservoir.
A paper entitled "The CO.sub.2 Huff 'N Puff Process" by Elmond L. Claridge presented at the "Enhanced Recovery Week Symposium: EOR Using CO.sub.2 ", Dec. 6, 1984, at Houston, Tex., contains a review of several publications which discuss the injection of CO.sub.2 into an oil well alternating with production of the same well by a process referred to as "huff and puff." The article indicates that the cylic CO.sub.2 injection process lowered viscosity of the mixture of CO.sub.2 and crude oil, swelled the crude oil in which CO.sub.2 was dissolved, provided a solution-gas drive achieved by dissolving CO.sub.2 in the crude oil up to a higher saturation pressure followed by lower pressures during production, and vaporization of components into CO.sub.2 with recovery of light crude oil components from produced CO.sub.2.
Burnett Pat. No. 3,841,406 discloses a method of recovering oil from an oil-bearing formation in which a gas having a limited solubility in oil is injected into the formation via a well, to increase the formation pressure, and thereafter a slug of carbon dioxide is injected via the same well. Following injection of the carbon dioxide, the well was shut in for a period of one to three days to permit the carbon dioxide to become dissolved in the oil to establish in the vicinity of the well bore a zone of oil saturated with carbon dioxide, so as to reduce the viscosity of the oil within a radial distance of about 20 to 100 feet from the well.
The technique of forming acids in situ with the oil or gas bearing strata in secondary and tertiary production wells in flooding projects are well known. Some examples of these techniques are described in U.S. Pat. Nos. 2,011,357; 3,072,185; 3,091,292; 3,259,187; 3,344,858; 3,353,579; 3,392,782; 3,398,791; 4,250,965 and 3,532,165.
The method disclosed in the patent to Wiseman significantly stimulates production. However, in certain well treatment processes it appears that the use of an acid which is stronger than carbonic acid is needed to further increase the permeability of the strata without subjecting the tubing string and casing to highly corrosive acidizing agents and attendant potentially ecologically damaging results of conventional acidizing processes.